This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a rotating polygon to a movable photoconductive member to record electrostatic latent images thereon, and, more particularly, to two point power control implemented by a microprocessor to digitally control the power from dual laser diodes.
In recent years, laser printers have been increasingly utilized to produce output copies from input video data representing original image information. The printer typically uses a Raster Output Scanner (ROS) to expose the charged portions of the photoconductive member to record an electrostatic latent image thereon. Generally, a ROS has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets.
The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan (process) direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper.
A difficulty in the past, however, is that prior art techniques in power control of laser diodes has been done with thermoelectric (TE) coolers that regulate the temperature of the laser to minimize power variation. These (TE) coolers are expensive, bulky in size, and very inefficient to operate. More recent methods employ analog power controls which become increasingly difficult to implement when controlling the newest technology lasers with multiple beams in the same package. When using a dual laser diode for simultaneous imaging, it is very important to balance the power of the two beams to provide uniform exposure. In addition, in prior art machines, exposure control has often been set by a control knob implemented with analog signal wires sensitive to noise.
Thus it would be desirable to provide a power system control that overcomes many of these difficulties in the prior art. It is therefore an object of the present invention to overcome not only changing characteristics due to temperature, but also differences between dual lasers in providing the necessary power balance. It is another object of the present invention to provide a microprocessor based digital control with embedded intelligence and diagnostic capability in controlling laser power. Another object of the present invention is to vary exposure setpoints by serial download of digital information including functional parameter data such as control loop compensation data. Other advantages of the present invention will become apparent as the following description proceeds, and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.